1. Field of the Invention
The present invention relates to a semiconductor device such as a power diode.
2. Description of the Background Art
FIGS. 8A and 8B are configuration diagrams of a heretofore known power diode 500, wherein FIG. 8A is a main portion plan view, and FIG. 8B is a main portion sectional view taken along the line X-X of FIG. 8A. The power diode (hereafter, simply called the diode) is a vertical diode.
The diode 500 includes an active region 11 formed of a p anode region 2, a p guard ring region 3 configuring a breakdown voltage structure 16 surrounding the active region 11, and an insulating film 4 (for example, an oxide film) which is a protection film on the p guard ring region 3. To describe in further detail, the diode 500 includes the p anode region 2 disposed in the front surface of an n semiconductor substrate 1, the p guard ring region 3 disposed so as to surround the p anode region 2, the insulating film 4 disposed on the p guard ring region 3, an n cathode region 6 disposed on the rear surface of the n semiconductor substrate 1, an anode electrode 7 disposed on the p anode region 2, and a cathode electrode 8 on the n cathode region 6. The p guard ring region 3 is configured of ring-shaped p+regions 3a, and a region of the n semiconductor substrate 1 in which no diffusion regions are formed forms an n drift region 1a. Herein, the diffusion regions refer to regions, such as the p anode region 2, p+regions 3a, and n cathode region 6, which are formed by diffusing or injecting impurities from or into the surface of the n semiconductor substrate 1.
When the diode 500 is put into an on state, holes are injected into the n drift region 1a from the p anode region 2, and electrons are injected into the n drift region 1a from the n cathode region 6 so as to neutralize the holes. Conductivity modulation thus occurs in the n drift region 1a, the on-resistance of the diode decreases, and on-voltage reaches a low value. Herein, a forward voltage drop of the diode is called on-voltage.
Also, when reverse current for cancelling forward current is caused to flow in order to put the diode 500 into an off state, the holes and electrons accumulated in the n drift region 1a are swept to the outside, reverse recovery current flows, reverse voltage is applied to the diode 500, and the diode 500 attains the off state.
Owing to the reverse voltage, a depletion layer expands from the pn junction of the p anode region 2 and n drift region 1a to the n drift region 1a. The depletion layer expanding laterally, on reaching the p guard ring region 3, further expands while leaping over the p+regions 3a configuring the p guard ring region 3, one p+region 3a after another. The width of the depletion layer expanding laterally increases in this way, meaning that field intensity on the surface decreases, and breakdown voltage on the surface is stably maintained.
Also, in JP-A-6-77506, in a power pin diode wherein an anode region is formed of a Schottky barrier junction and a pn junction, a p floating region is provided in an active region so as to be adjacent to the p anode region, and a MOS gate structure is provided between the p anode region and p floating region. At an on time, negative voltage is applied to a MOS gate, thus forming a p channel region, and the p anode region and p floating region are connected to expand an energization region, thus lowering on-voltage. When turning off the power pin diode, the MOS gate is turned off prior thereto, the p floating region is separated from the p anode region, and subsequently, reverse current for cancelling forward current is caused to flow, thus turning off the diode. The forward current decreases owing to the reverse current, excess holes and electrons are swept to the outside, and thus decrease. By further causing reverse current to flow, reverse recovery current flows through the pn junction and Schottky barrier junction, and excess holes and electrons are extinguished, thus attaining an off state. As the reverse recovery current in the Schottky barrier junction is low, reverse recovery current and reverse recovery loss decrease as compared with a power pin diode wherein the whole of an active region is formed of a pn junction. In this way, JP-A-6-77506 discloses measures whereby it is possible to lower both on-voltage and reverse recovery loss in the power pin diode.
However, in the heretofore known diode 500 of FIGS. 8A and 8B, in particular, when it is a high-voltage diode, the proportion of the p guard ring region 3 in a semiconductor chip increases, and when the area of the chip is the same, the active region 11 decreases in the same proportion, and the on-voltage rises.
Also, in JP-A-6-77506, as the floating region is provided in the active region, anode current flows to the floating region, which is not connected to an anode electrode, via the anode region. As the current flowing to the floating region flows to a conductivity modulation layer while flowing laterally through the floating region, the on-voltage rises as compared with when the whole of the active region is formed as the anode region. That is, with the structure of JP-A-6-77506, a conducting area decreases equivalently, as compared with when the whole of the active region is formed as the anode region.
Also, it is not described in JP-A-6-77506 that a guard ring provided in the outer peripheral portion of the active region is connected to the anode region, thus lowering the on-voltage.